1. Field of the Invention
The present invention relates generally to a system for cooling the air which is supplied to an engine by a turbocharger driven by the exhaust gas, and more specifically to a supply-air cooling system whereby when engine load rises beyond a predetermined level, the supply air is cooled before being pressurized by an air-supply blower.
2. Description of the Prior Art
In order to improve the fuel consumption rate of an automotive vehicle engine, a turbocharger has recently attracted attention in the industry, in which the power of the exhaust gas rotates an air-supply blower connected to a turbine for forcibly supplying air into the engine, in order to improve the efficiency in supplying intake air, that is, to improve the engine output.
Heretofore, in such an engine provided with a turbocharger as described above, temperature of the supply air pressurized by the air-supply blower rises as high as 150.degree. C. under heavy load in a steady state. Therefore, the density of the supply air is inevitably reduced due to an increase in temperature of the supply air, and, as a result, the efficiency in supplying intake air cannot rise to as high an extent as expected. In other words, there exists a problem such that it is impossible to improve the engine output as much as theoretically possible. Further, in the case of a gasoline engine, the higher the supply-air temperature, the more often engine knocking occurs.
To overcome this problem, there exists a simple prior-art cooling system by which pressurized supply air is cooled to increase the density of intake air before being introduced into the engine. This prior-art cooling system comprises only a cooling-fin unit, such as a radiator, to exchange heat.
In such a simple prior-art supply-air cooling device, however, since heat is absorbed by only the atmosphere and the difference in temperature between the heated supply air and the atmosphere is relatively small, it has been necessary to increase the area from which heat is emitted in order to obtain a sufficient heat exchange rate. In addition, there has been a need to increase the cross-sectional area of the duct through which the air is supplied, in order to reduce pressure loss due to flow resistance within the ducts. As a result, the size and volume of the cooling device itself inevitably becomes large, thereby increases in pressure of the air supplied to the engine are delayed.
Further, since the cooling device is disposed for downstream from the air-supply blower, significant pressure-resistance and leakage-resistance are required.
A more detailed description of the prior-art supply-air cooling system will be made hereinafter under DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS with reference to the attached drawing.